Semiconductor chip carriers (SCC), in addition to providing protection for the semiconductor chips, must also provide a means for physical attachment to other portions of a composite assembly of components, some means for coupling electrical signals into and out of the semiconductor chip, and some means for dissipating thermal energy generated by the semiconductor chip. Semiconductor technology continues to advance, creating denser, more complex and higher (IO) count semiconductor chips which generate more thermal energy. At the same time the electronics market is placing a premium on component assembly density. All of this has combined to dramatically drive semiconductor chip carrier IO density up.
Previous advances in technology have addressed in various ways all of the above problems, in isolation, for example, thermal dissipation in power semiconductors often dictates a package mounted by some form of threaded mechanical fastener or IO density via a TAB package. Other reasonable compromise solutions to all three of the above requirements for SCCs have been demonstrated previously.
Representative of the art today are various SCCs which are mounted by soldering, often via a solder reflow process, each IO contact pad to a respective contact pad on a substrate. With this approach the sum total of all solder joints provides the mechanical orientation and attachment to the substrate, to some extent a path for conducting thermal energy away from the SCC, and the requisite electrical coupling. This dependence on the solder joints for all three functions places severe constraints on these joints as well as the underlying characteristics of the substrate. Generally the solder joints must be strong and resilient enough and therefore large enough to accomplish these mechanical functions and thermal duties without failing or causing intermittent electrical coupling.
To achieve modest levels of joint density, the underlying substrates are usually carefully chosen to have temperature expansion characteristics closely matched to those of the SCC. This approach, at best a compromise, requires still more care to characterize temperature differences between the SCC and its substrate so as to avoid undue stress on the solder joints from differential expansion.
If all the above issues are carefully addressed the solder joint size can be somewhat reduced and therefore potential joint density increased at least to modest levels, at modest levels of thermal dissipation. As is readily observed this dependence on the electrical coupling means for all the requisite functions of an SCC severely constrains the breadth of circumstances and potential application of previously known SCCs.